Henotheism (n) is defined in the Oxford English Dictionary as the “belief in one god without denying the existence of others.” Or as Max Müller wrote in “Semitic Monotheism” (1881), “The latter form of faith, the belief in One God, is properly called monotheism, where as the term henotheism would best express the faith in a single god.”
The word itself is derived from the Greek heno- (from hen which in the neuter of heis, or one) + the(o)-+-ism and is commonly considered to be the stage of religious belief between polytheism and monotheism. To quote Tiele from Encyclopedia Britannica XX (1886) “From this primitive naturism sprang… henotheism, not the henotheism of Max Müller, or of Hartmann, or of Asmus, but a practical henotheism i.e. the adoration of one God above others as the specific tribal god or as the lord over a particular people, a national or relative monotheism.”